The long term objectives of this proposal are a better understanding of the mechanisms of visible light induced photoreceptor cell damage and of the processes of repair and recovery that occur following light exposure. We will study the influences of age and light rearing environment on retinal light damage in rats maintained for up to one year under carefully controlled conditions of weak cyclic light or darkness. Our focus will be on light initiated events in the nucleus and on visual cell responses that serve as markers of oxidation or that mark the photoreceptor cell for death. To examine the oxidative effects of intense light exposure in the retina, comparisons of light damage susceptibility will be made in rats of various ages, with or without antioxidant pretreatment. Standard techniques of continuous light exposure will be used, as well as exposure paradigms that accelerate the light damage process. We will test the ability of older rats to adapt to an abrupt change in rearing conditions and to resist light damage in comparison to young animals, following a change from weak cyclic light to darkness and vice versa. The temporal relationship between light initiated retinal damage and repair and recovery following light exposure will be studied to understand the events which lead to visual cell death or survival. Biochemical measures of visual cell loss, including rhodopsin analysis and visual cell DNA content, will be complemented by collaborative morphological evaluations of damage in the visual cells and in the retinal pigment epithelium. We will study the time course and interrelationships of DNA damage and repair, stress protein responses, mRNA induction and enzyme activities during light and during the dark recovery period following light exposure. This will be accomplished by Southern analysis of restriction enzyme fragments from genes expressed or not expressed in photoreceptors, by Poisson analysis of total retinal DNA strand breaks using ethidium bromide stained agarose gel electrophoresis, and by HPLC determination of oxidized guanosine nucleosides. Western, and Northern analysis of electrophoretically separated visual transduction and heat shock proteins and mRNAs will be used along with kinetic analysis of enzymes thought to be important for visual cell function. The specific aims of this proposal are: (1) To determine the effects of age and antioxidant (dimethylthiourea) treatment on specific molecules during damaging intense light exposure of rats reared for up to 12 months in weak cyclic light or in darkness. (2) To determine the effects of age and a change in light rearing conditions on specific molecules which can impact photoreceptor cell function and light damage susceptibility. (3) To determine the time course of visual cell repair and recovery from damage following exposure to intense visible light in rats with or without RPE damage. The characterization of light induced-rhodopsin mediated events in the photoreceptors of young and older rats may provide useful insights into the mechanisms of light damage in animals and into the relationship between long term light history and age related visual cell loss in humans.